1. Field of the Invention
The present invention relates to communication networks. More particularly, the present invention relates to the use of protocols such as spanning tree protocol (“STP”), rapid spanning tree protocol (“RSTP”) and multiple spanning tree protocol (“MSTP”) in communication networks.
2. Description of the Related Art
FIG. 1 depicts simple network 100 that includes layer 2 Ethernet bridges conforming to IEEE Std.™ 802.1D-2004 (IEEE 802.1D) or IEEE Std. 802.1Q-2003 (IEEE 802.1Q) 105. In this example, there are 9 bridges A-I and three stations, 140-142, connected via 12 local area networks (LANS) 125, 130, and 150 (10 instances). The letters A-I are Bridge IDs. Lower letters (e.g., A) are “better” than higher letters (e.g., D), according to the convention of IEEE 802.1D or IEEE 802.1Q, which are hereby incorporated by reference for all purposes. Port path costs 110 are indicated for each LAN. This diagram assumes that all bridge ports attached to the same LAN have the same value configured for their port path cost, though this is not required either by IEEE 802.1D or by the present invention.
Bridges create a spanning tree over network 100 by exchanging protocol packets called Bridge Protocol Data Units (BPDUs). Using these packets, the protocol state machines implemented in each bridge select certain ports to be part of the active topology of the network and certain others to be blocked. (For the purposes of this invention, STP and RSTP are equivalent; in any context where STP is mentioned, RSTP is equally applicable.) The spanning tree is “spanning” in the sense that all LANs are connected. It is a “tree” in that there is exactly one path between any given pair of bridges or stations.
According to STP, one of the bridges of network 100 (in this example, bridge A) will be elected as the “Root Bridge”. The tree is constructed by each bridge selecting the port that is closest to the Root Bridge as its “Root Port,” where “closest” is defined as the path to the root bridge with the least numerical sum of root path costs for the bridge ports traversed. (Only ingress from a LAN to a bridge counts in this summation, not egress from a bridge to a LAN.) Where two or more paths have the same sum of port path costs, tiebreaker values are used. The tiebreaker values are Bridge IDs and Port IDs, as defined in IEEE 802.1D and IEEE 802.1Q. Bridge E selects from among three equal cost paths, thus breaking three potential spanning tree loops, by using the bridge IDs of the adjacent bridges to select port 117 as its root port, and to mark as “alternate ports” and block ports 115 and 120. (Although these terms can have different meanings when used by those of skill in the art, the terms “packet” and “frame” will sometimes be used interchangeably herein.)
Although STP provides for the orderly flow of packets, it does not allow for all LANs in a network to be used in an optimal fashion. There is never a path from any bridge or station in the network to the root that has a lower sum of path costs than the path along that root bridge's spanning tree, but paths between other pairs of bridges or stations may not be optimal. For example, if ports 115 and 120 are blocked, the path from station 142 to station 141 is 142-E-F-I-A-B-D-G-141, for a total cost of 11 (station 141 does not count the cost of the last hop), instead of the much shorter path 142-E-G-141, cost 3. It would be desirable to address this limitation of the prior art.
MSTP offers an improvement over STP in that multiple spanning trees are created. Network 100 could be configured to have up to 64 spanning trees. For example, each bridge A through I could be configured to be the Regional Root Bridge of a separate multiple spanning tree instance (MSTI). IEEE 802.1Q bases the determination of the MSTI to which a given frame is assigned, and over which that frame is forwarded, solely upon the frame's Virtual Local Area Network Identifier (VLAN ID or VID), which in turn, is based upon only a) which port the frame entered the network, and/or b) what higher-layer protocol the frame carries. VIDs can often be assigned to ports and/or protocols that result in the selection of suitable MSTIs. However, this is not always possible, since any given station connected to the network may converse with a number of other stations, and in turn, they with others. For example, in network 100, each station 140, 141, and 142 may converse with each other. It would be desirable to always be able transmit a frame along the most direct path to its destination in all cases.